Everyone should know about the adventures of Lewis and Clark when the explored the West looking for the Northwest Passage. Once you've read a solid history of their trip, like Stephen Ambrose's Undaunted Courage, this makes a great companion book. It tells the story of the expedition from point of view of Seaman, the black Newfoundland dog who accompanied the explorers and who was a valued companion to the men--particularly to Meriwether Lewis.
The story is built around the framework of the actual chronology of the trip, with accurate dates and many historical figures. The author skillfully weaves in a few fictional people and events to create a more interesting narrative, but nothing that conflicts with the record. Much of the time you forget that it's a dog narrating, until some delightful take on an event is described which emphasizes smell or hearing instead of the way that a human would have perceived things.
This book is written for young adults but it's not juvenile. It's fast-paced, with colorful prose and though-provoking descriptions. I wouldn't recommend it as a stand-alone book for someone who doesn't already know something about Lewis and Clark; but it makes a great supplement and a fascinating way to look at their achievement from another angle. Also, the author is able to ascribe personality traits to many of the main characters that really help them stand out--like John Colter, George Drouillard, Toussaint Charbonneau, and the Shoshones Old Toby and Twisted Hair (oddly, I thought William Clark got a little short shrift in that regard). Sacagawea (whom Seaman calls Bird Woman, supposedly the meaning of her name in the Hidatsa language) is already well enough known to even casual readers of history that this story doesn't add much to her luster.